Unlike the generic Sub Form elee community edition, AJAX Sub Form gives you the flexibility of dynamically populating a subform as the value it watches changes.
The example below shows how the AJAX Sub Form changes its content depending on the field value it's tied to.
Figure 1: Configuration of AJAX Sub Form Element
Figure 2: AJAX Sub Form Element Tied to Multi-Select Form Element
Figure 3: Content Change in AJAX Sub Form During Field Value Change